The continuous evaporation of fluid circulating through a cooling tower will precipitate increasing concentrations of dissolved solids in the unevaporated fluid. Eventually, as the fluid is continuously recycled back to the cooling tower, the concentration of the dissolved solids will become so high that the fluid will no longer be able to function as a coolant.
Typically, where the coolant is water, it is brought into the cooling tower at a temperature which may be as high as 180.degree. F. or more, a large fraction, say one third of the water, is evaporated during each pass through the tower as the latent heat of evaporation of the remaining water is dissipated in the atmosphere. Since only water can evaporate and none of the dissolved solids which are present in it can evaporate, the concentration of the dissolved solids will increase as the water passes from the cooling tower to then serve as a coolant and then be recycled to the tower.
IT IS TRUE THAT FRESH WATER IS ADDED TO MAKE UP FOR THE WATER THAT IS EVAPORATED BUT THE MAKE-UP WATER ALSO CONTAINS DISSOLVED SOLIDS SO THAT GRADUALLY THE CONCENTRATION OF THE SOLIDS WILL RISE TO A LEVEL WHICH WILL PREVENT THE WATER FROM SERVING AS AN EFFICIENT COOLANT.
The solids which are objectionable and which must be removed include trivalent chromium, hexavalent chromate, molybdate and bivalent zinc. The chromium and zinc can be removed from solution by contacting the solution with a weak or strong acid cation exchange resin. The chromate and molybdate can be removed by contact with a weak base anion exchange resin in the presence of acid. Each of the resins requires a different regenerant so that the process will require bulky equipment due to the requirement for separate sites for the regeneration of the two resins. In the event that it is desired to remove only chromate and/or molybdate only the weak base anion exchange resin is necessary, but acid must be added to the solution if the chromate or molybdate is to be removed. Thus pollutant is removed but another pollutant is added to its solution.
The foregoing is one example where the present invention could be used. It could be used in other applications where it is desired to remove solids because they are pollutants or because they are valuable. Representative of such applications are those which involve the removal of molybdate values from mine tailings or the removal of contaminating hexavalent chromate values from cooling tower blowdown, especially where these contaminated waters do not contain any appreciable quantities of contaminating trivalent chromium and zinc values.